The present invention relates to the fabrication of electroceramic materials, such as lead zirconate titanate and barium strontium titanate, of high sintered density at a lowered sintering temperature. More particularly, the invention advantageously provides a method for the production of electroceramics of high density from coprecipitated hydroxides and oxalates without the use of any sintering aids/additives. The direct processing technique also advantageously offers a unique advantage in minimizing the level of contamination in the sintered electroceramics, due to the elimination of certain intermediate processing steps, such as the calcination and subsequent ball milling of precursor powders.
Electroceramics, such as lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT) and barium strontium titanate (BST), are technologically important materials in electronics and microelectronics due to their unique piezoelectric, ferroelectric and many other electromechanical, pyroelectric and optoelectrical properties. On the one hand, high sintered density and uniform microstructure are among the most desirable features for almost all the electroceramics in achieving many of these desirable electrical properties. It is however difficult to achieve a sintered density close to the theoretical density for most of the electroceramic materials via conventional ceramic processing routes using mixed oxides as the starting materials. This, together with many of the undesirable features of sintered electroceramics, such as non-stoichiometry and wide fluctuations in composition and microstructure, are to a large extent due to inadequate processing routes chosen for fabricating these materials. For example, as a result of the high volatility of PbO at elevated temperatures, there is a significant loss of lead oxide in the production of lead-containing electroceramics such as PZT, PLZT and lead magnesium niobate (PMN) at high sintering temperatures of  greater than 1200xc2x0 C., leading to the formation of one or more pyrochlore phases and therefore a reduction in sintered density. On the other hand, the use of electroceramic materials in co-firable mulitlayer electronic and microelectronic devices for many electromechanical applications requires that the electroceramic compositions be sinterable at temperatures below 1000xc2x0 C., as a lowered firing temperature will apparently alleviate, if not completely eliminate, the detrimental interactions between the ceramic layer and the electrode layer seen at a higher sintering temperature. It is envisaged that a reduction in the sintering temperature of electroceramic materials will eventually lead to the substitution of very expensive electrodes, such as platinum and palladium, by much cheaper ones such as silver, nickel, copper and their alloys. Therefore, there is considered to be substantial technological and economic significance in methods of lowering the sintering temperature of electroceramics without sacrificing the electrical properties thereof too greatly.
Two general approaches have been taken in order to lower the sintering temperature of electroceramics, preferably to the range of  less than 1000xc2x0 C. for PZT as an example. These include (i) employing an ultrafine starting powder prepared mainly via various chemistry-based processing routes; and (ii) using a sintering aid/additive of low melting point, such as V2O5, Bi2O3, and an eutectic mixture of CuO and barium or strontium oxide, as claimed by Buchanan and Wittmer in U.S. Pat. No. 4,283,228 (1981) and Srivastava, Bhalla and Cross in U.S. Pat. No. 5,433,917 (1995), respectively. Unfortunately, many of the chemistry-based powder preparation routes for electroceramics are associated with disadvantages such as very high manufacture cost and low production yield, which make them unsuitable for industrial scale production of electroceramic components. Furthermore, most of these have yet to demonstrate any significant advantages in lowering the sintering temperature of electroceramic materials over the conventional electroceramic processing routes. The approach of employing sintering aids/additives is attractive in terms of being able to lower the sintering temperature of electroceramics by forming a liquid phase at the grain boundaries and grain junctions, as has been demonstrated for PZT to below 1000xc2x0 C. However, the sintering aids/additives are often detrimental to the electrical properties of the electroceramics, due to the formation of a secondary non-electroceramic phase concentrated at the grain boundaries and grain junction of sintered electroceramics. In addition to this, the majority of the sintering aids/additives suggested are extremely toxic and therefore are very difficult to handle in any large scale production of electroceramic components.
Using high-purity inorganic or organic salts as the starting materials, precursor-calcination-milling-pelleting-sintering is the well established fabrication route for many electroceramics. The precursor is normally prepared via a wet-chemistry route, such as sol-gel, hydrolysis, hydrothermal reaction or coprecipitation, followed by calcination and ball milling steps in order to form the required electroceramic phase and to adjust the powder characteristics. The calcination of precursor powders at an intermediate temperature unfortunately removes almost all the advantages offered by most of the chemistry-based powder preparation techniques, including very high specific surface area, ultrafine particle size and narrow particle size distribution. This is a result of particle agglomeration in the calcined electroceramic powders. The presence of hard particle agglomerates adversely affects subsequent compaction and sintering behaviour of ceramic powders and results in a reduced density and the occurrence of microstructural defects in sintered electroceramics, as observed by F. F. Lange, see J. Amer. Ceram. Soc., 66, pp.396-398 (1983) and W. H. Rhodes, See J. Amer. Ceram. Soc., 64, pp. 19-22 (1981). A post-calcination milling process is generally required in order to modify the powder characteristics, e.g., the particle/agglomerate size and particle morphology, before the powder is shaped to a powder compact or a component shape and then densified at high sintering temperatures. However, some hard particle agglomerates (aggregates) can not be effectively eliminated by conventional milling such as ball milling. Furthermore, a mechanical milling inevitably introduces contamination into the electroceramic materials. Contamination in the range of 0.1 to 2 wt % is common under normal milling conditions as pointed out by Moulson and Herbert (Electroceramics, Chapman and Hall, London, 1990). Such high levels of contamination are unacceptable for many electroceramic materials.
Reaction sintering, in which the reaction between constituent component phases occurs concurrently with the densification process at the sintering temperature, has been established as a viable fabrication technique for a wide range of oxide and non-oxide ceramics. It offers at least two advantages over the conventional ceramic processing routes: (i) no intermediate milling and drying of pre-reacted and post-reacted compounds are required; and (ii) the free energy associated with the reaction helps facilitate densification. For example, T. R. Shrout, P. Papet, S. Kim, and G. Lee, see J. Amer. Ceram. Soc., 73, pp.1862-1867 (1990), and S. Kim, G. Lee, T. R. Shrout and S.Venktanari, see J. Mater. Soc., 26, pp.4411-4415 (1991), observed that the densification of PZT was enhanced by the reaction of constituent oxides in a partially reacted system. Since the precursors exhibit a higher degree of mixing homogeneity, the reaction may be completed at a lower temperature than those for mixed oxides.
The present invention relates to the fabrication of electroceramic materials of high sintered density at sintering temperatures of substantially lower than those normally required by the traditional precursor-calcination-milling-pelleting-sintering route, without use of any sintering aids/additives.
Accordingly, the present invention provides a method for producing an electroceramic material from hydroxide and/or oxalate precursors, said method comprising:
compacting a mixture containing at least one hydroxide and/or oxalate precursor to form a preform;
thermally treating the preform at a predetermined temperature;
re-compacting the preform; and
sintering the re-compacted preform to form said electroceramic material.
The invention also provides an electroceramic material produced by the method of the immediately preceding paragraph.
In a preferred embodiment, the invention involves the use of fine and reactive hydroxide and oxalate precursors, preferably prepared via coprecipitation from aqueous solutions containing desirable cations. The precursors are not calcined at an intermediate temperature in order to avoid the adverse effects of particle agglomeration on densification of electroceramics at the sintering temperature. Instead, they are directly shaped into pellets or any other complex shapes by compaction. The pellets or preforms of hydroxide or oxalate precursors are then thermally treated in a controlled manner preferably at a temperature in the range of 400xc2x0 C. to 700xc2x0 C., followed by a re-compaction, preferably by isostatic pressing to further increase their green densities. Sintering of the isostatically pressed compacts is carried out at a temperature which may advantageously be xcx9c200xc2x0 C. lower than that required in traditional precursor-calcination-milling-pelleting-sintering routes
Compared with conventional precursor-calcination-milling-pelleting-sintering route, the intermediate calcination and milling steps of precursor powders are advantageously eliminated and a lower sintering temperature may advantageously be used in the present method. Using the present method, hydroxide-derived lead zirconate titanate (PZT) may be sintered to a relative density of  greater than 98% theoretical at temperatures below 1000xc2x0 C. without the use of any sintering aids/additives. Similarly the oxalate-derived barium strontium titanate (BST) may be sintered to a density of  greater than 99% theoretical density at a temperature of 1200xc2x0 C. for 1 hour. These sintering temperatures are, as discussed above substantially lower than those required by conventional precursor-calcination-milling-pelleting-sintering routes.